Pericyclic Reactions: Electrocyclic Reactions

Electrocyclic Reactions: Examples for 6-Electron Systems

6-Electron systems (hexatriene / 1,3-cyclohexadiene)

Vitamin D3 is formed in the skin by uv light; lack of vitamin D causes rachitis. The
first step in the formation of vitamin D3 is an electrocyclic ring opening:

Fig.2

Electrocyclic ring opening in the synthesis of vitamin D3

Electrocyclic 6-electron systems also found applications as molecular switches or as
photochromous compounds, respectively, for example:

Fig.3

Electrocyclic reaction as a molecular switch

A colorless solution of spiropyrene turns blue upon irradiation. The reverse reaction
can be induced thermally and occurs within a few seconds.

Cycloheptatriene and cyclooctatetraene are in an equilibrium with small amounts of
their corresponding isomers formed by electrocyclic ringclosure. The equilibrium is shifted
nearly completely to the left side because of the high ring strain in 3- and 4-membered
rings.